As well known, in conventional gas turbines, acoustic oscillation usually occurs in the combustion chambers of the gas turbines. With the term chamber is intended any gas volume where combustion dynamics occur. In such chambers the flow of a gas (for example a mixture of fuel and air or exhaust gas) with high velocity usually creates noise. Burning air and fuel in the combustion chamber causes further noise. This acoustic oscillation may evolve into highly pronounced resonance. Such oscillation, which is also known as combustion chamber pulsations, can reach amplitudes and associated pressure fluctuations that subject the combustion chamber itself to severe mechanical loads that may decisively reduce the life of the combustion chamber and, in the worst case, may even lead to destruction of the combustion chamber.
To reduce the acoustic oscillations noise it is well known in the art to install acoustic damping devices like Helmholtz resonators.
Typically, these kinds of dampers are physical devices that are often positioned around the combustion chamber (on the liner, on the front panel). They usually include an empty volume (where air can flow) and a neck that connects the volume to the combustion chamber.
The resonance frequency and damping power of a Helmholtz damper depends on its geometry and on the flow through its neck. The maximum dimensions of a Helmholtz damper to be used in a gas turbine can be limited due to geometrical constraints imposed by the section where the damper needs to be mounted. A particularly stringent constraint consists of the maximum length of the neck, as the latter is one of the key parameter which affects the damping capabilities of such device. Limitations in the neck length limit the damper effectiveness, in terms of frequency that can be targeted and damping.
However, if the desired length of neck, selected in order to achieve the most suitable frequency associated to the operative conditions of the machine, is longer than what is geometrically allowed (taking into consideration the available space around the combustion chamber), the solution generally adopted is to narrow the neck diameter. Nevertheless, such solution inevitably decreases the damper efficiency.